1. Field of the Invention
The present invention generally relates to composite material usage for acoustic energy absorption in structural applications and, more particularly to composite to metal joints.
2. Description of the Prior Art
Previous studies have demonstrated success in design, analysis, and hydrotesting of composite cylinders for pressure hull applications. One reason for use of composites in pressure hulls is reduced acoustic signature compared to metallic hulls. To fully exploit this advantage it is necessary to consider acoustics and vibration damping when designing pressure hull joints. It is assumed that without consideration of these factors, a typical structural pressure hull joint would serve to focus and radiate acoustic energy, minimizing the advantage gained through use of composite materials.
Composite materials are seeing increased usage in structural applications. One class of forms are cylindrical sections which are being considered for underwater applications. In all cases, the composite material is adhesively bonded and/or mechanically attached to metallic couplings. These couplings are then attached to one another using a variety of methods through the metallic couplings.
Many of the potential benefits of constrained layer damping, and of the inherent material damping behavior of composites, can be lost through joining of the composite shell to a metallic joint ring. This is due to the fact that acoustic energy is focused at the joint ring, and the ring itself provides an excellent radiator of acoustic noise.
The composite to metal joint is in general designed with the sole intent of meeting the structural requirements of the application. This typically means that no provisions exist to mitigate the transfer of mechanical or acoustic energy, that may be generated in the composite, from propagating into the metallic coupling. The energy that is transferred to the metallic coupling can then be efficiently radiated into the surrounding fluid of the above mentioned applications.
Composite materials, when used for structural applications, are typically joined using metallic end fittings. Although the composite has an inherent material characteristic of high energy dissipation, the metallic coupling to which it is attached does not. These systems, then, which utilize innovative design concepts for energy dissipation by the composite, have a highly efficient nondamping component, the metallic coupling, integral to it, which results in a system which will radiate noise and vibration efficiently, thereby negating the design advantages of the composite element. This application will describe the combination of viscoelastic damper and plastically deformable adhesive to produce an energy absorbing joint which may sustain high preload.